Basic Planetary Gear Set
A planetary gear set contains three gears that remain in mesh at all times. The transmission can change the gear ratio by driving one member of the gear set while holding another. This causes the third member of the gearset to be the driven gear.
Sun Gear: The sun gear, similar to the sun in our solar system, is located at the center of the planetary gear set. This gear remains in constant mesh with the pinion gears.
Ring Gear: The ring/annulus gear has internal teeth and remains in constant mesh with the pinion gears. It's the set's outer gear and remains in constant mesh with the pinion gears.
Pinion Gears: The pinion carrier is located between the ring and sun gear. It remains in constant mesh with the sun and the ring gear. It consists of several pinion gears contained inside of a carrier called the planet carrier. This carrier contains the needle bearings and pins that the pinion gears ride on. Manufacturers can add a pinion gear to a carrier to increase its strength.